Aqueous dispersion containing pyrogenically prepared metal oxide particles and dispersants

ABSTRACT

An aqueous dispersion containing pyrogenically prepared metal oxide particles and dispersants An aqueous dispersion containing pyrogenically prepared oxide particles of titanium, zinc, iron or cerium with an average particle size, expressed as the median, in the dispersion of less than 250 nm, that the particle sizes of the oxide particles in the dispersion are not distributed symmetrically and the dispersion contains, as a dispersant, at least one compound of the formula (I) and/or a maleic anhydride/acrylate copolymer of the general formula (IIa) and/or a maleate/acrylate copolymer of the general formula (IIb). It is prepared by dispersing a predispersion stream by means of a high energy mill. It can be used in sunscreen formulations.

The invention provides an aqueous dispersion containing pyrogenicallyprepared metal oxide particles and a phosphate and/or maleicanhydride/maleate-acrylate copolymer as dispersant, a process forpreparing this dispersion and its use to prepare cosmetic formulations,in particular sunscreen formulations.

To protect the skin from too intense UV radiation, cosmetic preparationswhich contain UV filters, such as cremes or lotions, are used, thesebeing largely transparent on the skin and pleasant to apply.

As UV filters, they contain one or more organic compounds which absorbin the wavelength region between 290 and 400 nm: UVB radiation (290 to320 nm); UVA radiation (320 to 400 nm).

Energy-rich UVB radiation causes the typical symptoms of sunburn and isalso responsible for suppressing the immune defence system, while UVAradiation, which penetrates further into the layers of skin, causespremature ageing of the skin. Since the combined effects of the twotypes of radiation can encourage the production of skin diseases causedby light, such as skin cancer, the search for possibilities to improvethe degree of TV protection already produced even more significantlytherefore started at an early stage.

It was found that ultrafine pigments based on metal oxides can alsoscatter, reflect and absorb UV radiation. Therefore highly disperseformulations of these are an effective supplement to the organic UVfilter in suncreens.

Thus, ultrafine titanium dioxide is used in many different ways incosmetic formulations because it is chemically inert and toxicologicallyacceptable and does not lead to either skin irritations or tosensitisation. Currently, it is the most frequently used and mostimportant mineral light-protective substance. In addition to titaniumdioxide, ultrafine zinc oxide is being used to an increasing extent.

Coarsely divided materials (pigments) and finely divided materials(micropigments) should be differentiated. In the case of micropigments,the average primary particle size is generally well below 200 nm, mostlyin the range 10 to 100 nm, generally below 50 nm.

The primary particles are the smallest particles which are producedduring preparation of the pigments. Primary particles may be present inthe form of individual crystallites or else in the form of severalcrystallites which have densely intergrown with each other via faces.Aggregates are particles which consist of several primary particles,wherein the primary particles have intergrown with each other in atwo-dimensional array. An agglomerate is understood to be an associationof primary particles or aggregates which are held together by attractiveforces such as, for example, hydrogen bridge bonds.

Coarsely divided pigments (0.2 to 0.5 μm) absorb and/or reflect broadlyand relatively evenly over the entire UV region and the region ofvisible light, whereas finely divided material exhibits a clear increasein effect in the UV region with a simultaneous loss of effect in thelongwave TVA and in particular in the visible region. Since only verylittle of the visible light is reflected, preparations based on theseactive substances are therefore largely transparent.

A reduction in photochemical activity can be produced by inorganic andorganic surface components such as, for example, Al₂O₃, SiO₂ and/orfatty acid (salts), siloxanes. These substances can adhere to thesurface as a result of chemical or physical sorption (latticedoping/coating).

In transparent cosmetic formulations, it is important that the particlesare as small as possible so that they cannot be detected on the skinwith the naked eye. At the same time, the UV protective effect of asunscreen should not be reduced and the particles should not settle outduring storage.

For this purpose, the aggregated and agglomerated metal oxide particlesare dispersed. This process is understood to include the incorporation,reduction in size and uniform distribution of solids in a liquid phase.

In practice, it has been shown that dispersion problems increase withincreasing degree of fineness of the particles so that the dispersionprocess overall is one of the most costly sub-steps in the production ofcosmetic formulations. The challenge in practice, therefore, is toseparate out the most costly part of the dispersing process, breakingdown the agglomerates, from production of the actual cosmeticformulations and to provide stable aqueous dispersions with the highestpossible concentration of ultrafine metal oxide particles, thesepreferably being of low viscosity or at least still pumpable or capableof flowing.

An essential feature of a dispersion is the size of the dispersedparticles in the dispersion. This size is called the secondary particlesize and describes primary particles, aggregates and agglomerates in thestate that they are present in the dispersion. In contrast to datarelating only to the primary particle size, information about thesecondary particle size reflects the actual situation in the dispersionand in sunscreen formulations.

It is possible to break down the agglomerates and wet the newly createdsurfaces with the aid of dispersing devices such as dissolvers, ballmills and rotor-stator machines, wherein breaking down depends on theenergy introduced. The energy of these dispersing devices is notsufficient for the very fine secondary particle sizes required forcosmetic and sunscreen formulations.

Although dispersing devices with higher energy inputs are known, such ashigh-energy mills, the disadvantage of these devices is the risk ofloosening the external layer in the case of coated particles. On theother hand, it is feared that organic dispersants, which are added tostabilise the dispersion, might be thermally decomposed by the energyinput and thus adversely modify the properties of the dispersion.

EP-A-876 841 describes the preparation of a titanium dioxide dispersionusing a high energy mill, wherein the average particle size in thedispersion is 0.16 μm. The dispersion is stabilised by adding aceticacid. Due to the odour, this type of stabilisation is unsuitable forcosmetic applications. On the other hand, the stability expected in theregion relevant to cosmetic applications, pH about 4.5 to 7.5, isregarded as low because it is in the vicinity of the isoelectric pointfor titanium dioxide. Although TiO₂ particles with sizes of this orderof magnitude scatter UV radiation very well, the small size of theparticles also leads to an undesired rise in photocatalytic activity.

Thus, the object of the present invention was to prepare a highlyconcentrated aqueous dispersion of ultrafine metal oxide particles witha comparatively low viscosity which is stable in the physiologicallyacceptable pH range of 4.5 to 7.5 and which has reduced photocatalyticactivity as compared with that of the prior art.

The object is achieved by an aqueous dispersion containing pyrogenicallyprepared oxide particles of titanium, zinc, iron or cerium with anaverage particle size, expressed as the median, in the dispersion ofless than 250 nm, characterised in that the particle sizes of the oxideparticles in the dispersion are not distributed symmetrically and thedispersion contains, as a dispersant, at least one compound of thegeneral formula I

in which

-   -   R represents an optionally branched, optionally multiple bond-        and optionally hydroxyl group-containing alkyl group with 6 to        22 carbon atoms,    -   A represents an ethylene group, propylene group, iso-propylene        group or butylene group,    -   M represents H, an alkali metal or an ammonium ion,    -   a is 0 to 30,    -   b is 0 to 2    -   and/or at least one copolymer of the general formula IIa        and/or at least one copolymer of the general formula IIb        in which, for IIa and IIb    -   M represents hydrogen, a monovalent or divalent metal cation, an        ammonium ion or an organic amine group,    -   a is 1 or, in the event that M is a divalent metal cation, a=0.5    -   X represents either —OM_(a) or —O—(C_(p)H_(2p)O)_(q)—R¹, where        -   R¹═H, or an aliphatic hydrocarbon group with 1 to 20 carbon            atoms,        -   a cycloaliphatic hydrocarbon group with 5 to 8 carbon atoms,            an optionally substituted aryl group with 6 to 14 carbon            atoms,        -   p=2 to 4, q=0 to 100,        -   —NHR² and/or —NR² ₂, where R²═R¹ or —CO—NH₂    -   Y represents O, NR²    -   A¹ represents an ethylene group, a propylene group, an        iso-propylene group or a butylene group,    -   m is 10 to 30,    -   n is 0 to 50,    -   k is 10 to 30, wherein the sum    -   m+k is within the range 20 to 60, preferably 20 to 40.

The wording not symmetric is understood to mean that the arithmetic meanof the distribution is greater than the median.

The mean is understood to be the arithmetic mean of the volume-weightedparticle size distribution. The median is understood to be the d₅₀ valueof the volume-weighted particle size distribution.

The asymmetric distribution of the particle sizes in the dispersionaccording to the invention is surprising. When dispersing pyrogenic,aggregated metal oxide particles, a symmetric normal distribution wouldbe expected to be present, this being recognised when the ratio of themean and the median values of distribution is 1.

Asymmetric distributions are understood to be, in addition to “skewed”monomodal distributions, also multimodal distributions.

Thus, for example, the dispersion of pyrogenically prepared aluminiumoxide using high dispersion energies leads to a symmetric normaldistribution. The asymmetric distribution of the dispersion according tothe invention means that a large proportion of the particles has thefineness desired for cosmetic applications, whereas a smaller proportionof coarser particles has a beneficial effect on the stability andrheology of the dispersion.

Pyrogenically prepared oxide particles of titanium, zinc, iron andcerium include those which arise from flame hydrolysis and also thosewhich arise from a flame oxidation process. In the case of flamehydrolysis, precursors of the metal oxides, for example metal halides ororganometallic compounds, are burnt in a hydrogen/oxygen flame, whereinthe precursors are hydrolysed. This synthetic method, originallydescribed for pyrogenic silicon dioxide, can also be used, for example,for titanium dioxide. In the case of flame oxidation, metal vapour isgenerally oxidised to the metal oxide in an atmosphere of oxygen. Theoxidation of zinc vapour to zinc oxide may be mentioned by way ofexample. From a toxicological and dermatological point of view, harmlesscompounds such as cerium oxide, zinc oxide, iron oxide and in particulartitanium dioxide are suitable for cosmetic formulations. Furthermore,the oxide particles also include mixed oxide particles, doped particlesor coated particles of titanium, zinc, iron and cerium with each otherand/or with silicon and/or aluminium. The BET surface area of the oxideparticles may vary over a wide range, from 5 to 200 m²/g.

The surfaces of the previously mentioned metal oxide particles may alsobe further modified with organic compounds. Water-repellent metal oxideparticles can be obtained in this way. Examples of metal oxide particlesmodified with organic compounds are described, for example, in DE-A-4202 695, EP-A-1 078 957, EP-A-924 269, EP-A-722 992.

The metal oxide particles to be used according to the invention may be,for example, commercially available products which are obtainable underthe relevant trade names, also with inorganic or organic coatings, suchas for example Micro Titanium Dioxide MT 100 AQ and MT 150 W(Tri-K-Tayca), UV-Titan M 212 (Kemira), and titanium dioxide P-25(Degussa).

Titanium dioxide T 805 (Degussa) has proven especially advantageoushere. Titanium dioxide T 805 consists, crystallographically of about 80%anatase and about 20% rutile and is coated with trialkoxyoctylsilane.It.is characterised by decreased photoactivity, reduced surfaceactivity, high cosmetic acceptability and very good water-resistance.

The dispersion may contain 20 to 60 wt. %, preferably 30 to 50 wt. %, ofmetal oxide particles.

The phosphates used according to the invention are represented inidealised form by the general formula (I)

As a result of the industrial method of preparation, mixtures arepresent in which the desired main components, according to the inventionpreferably the monoester and diester, are mainly present as well assmall proportions of the other possible reaction products.

They are prepared by reacting fatty alcohols R—OH or fatty alcoholalkoxylates R—O-(AO)_(a)—H with phosphoric acid or its derivatives byknown processes.

The fatty alcohols being used can be prepared by well-known processes byreducing fatty acids or their esters in the presence of catalysts. Inthe case of direct hydrogenation, fatty alcohols from triglycerides byreaction with hydrogen on a Cu/Cr catalyst in a one-step process in atubular reactor, wherein the fatty alcohol, 1,2-propanediol and waterare produced as the reaction products. In another process, a fattyalcohol is prepared from triglycerides via a transesterification stepfollowed by hydrogenation of the fatty acid ester.

Fatty acids which may be used, individually or as mixtures, are fattyacids such as n-caprylic acid, capric acid, 2-ethylhexanoic acid, lauricacid, myristic acid, palmitic acid, palmitoleic acid, isostearic acid,stearic acid, hydroxystearic acid (ricinoleic acid), dihydroxystearicacid, oleic acid, linoleic acid, petrolesic acid, elaidic acid,arachidic acid, behenic acid and erucic acid, gadoleic acid and thetechnical grade mixtures produced during the pressurised cracking ofnatural fats and oils such as oleic acid, linoleic acid, linolenic acid,and in particular rape-seed oil fatty acid, soy oil fatty acid,sunflower oil fatty acid, tall oil fatty acid. In principle, all fattyacids with similar chain distributions are suitable.

The concentration in these fatty acids or fatty acid esters ofunsaturated fractions is, unless this is required, adjusted down to adesired iodine value by the well-known catalytic hydrogenation processor is produced by mixing fully hydrogenated with non-hydrogenated fattycomponents. The iodine value, as a measure of the average degree ofsaturation of a fatty acid, is the amount of iodine which is taken up by100 g of compound in order to saturate the double bonds.

Preferably used are the alcohols from partially cured C_(8/18)-coconutor palm oil fatty acids, rape-seed oil fatty acids, sunflower oil fattyacids, soy oil fatty acids and tall oil fatty acids with iodine valuesin the range about 80 to 150 and in particular the alcohols fromtechnical grade C_(8/18)-coconut fatty acids, wherein a choice ofcis/trans isomers such as the elaidic acid-rich C_(16/18)-fatty acidfraction may optionally be of advantage. They are commercially availableproducts and are offered by a variety of companies under theirparticular trade names.

In addition to fatty alcohols, in particular guerbet alcohols and theiralkoxylates may also be used.

The alcohol alkoxylates R—O-(AO)_(a)—H may be obtained by well-knownprocesses by the addition of alkylene oxides in the presence of acid orbasic catalysts.

The -(AO)_(a)— group here represents groups such as ethylene oxide,propylene oxide, butylene oxide and/or tetrahydrofuran, preferablyethylene oxide, wherein a represents an average value of up to 30,preferably 3 to 15 units.

In the general formula -(AO)_(a)— represents either a homopolymer of oneof the alkylene oxides mentioned or block copolymers or copolymers withrandom distribution of two or more of the monomers in the polymermolecule.

In phosphates of the general formula I, R may advantageously be a fattyalcohol group with 6 to 22 carbon atoms, preferably 12 to 18 carbonatoms and a has a value between 1 and 30, preferably 3 to 15.

In phosphates of the general formula I, R may represent a guerbetalcohol group with 6 to 22 carbon atoms, preferably 12 to 18 carbonatoms and a has a value between 1 and 30, preferably 3 to 15.

These products are commercially available. They are used in amounts of0.5 to 30% with respect to the aqueous dispersion, preferably 3 to 15%with respect to the aqueous dispersion.

In the copolymers of the general formula IIa and IIb being used inaccordance with the invention,

the following definitions apply:

-   -   M represents hydrogen, a monovalent or divalent metal cation, an        ammonium ion or an organic amine group    -   a is 1 or, in the event that M is a divalent metal cation, a=0.5    -   X represents either —OM_(a) or —O—(C_(p)H_(2p)O)_(q)—R¹ where        -   R¹═H or an aliphatic hydrocarbon group with 1 to 20 carbon            atoms,        -   a cycloaliphatic hydrocarbon group with 5 to 8 carbon atoms,            an optionally substituted aryl group with 6 to 14 carbon            atoms,        -   p=2 to 4, q=0 to 100,        -   or —NHR² and/or —NR² ₂ where R²═R¹ or —CO—NH₂    -   Y represents O, NR2    -   A¹ represents an ethylene group, a propylene group, an        iso-propylene group or a butylene group,    -   m is 10 to 30,    -   n is 0 to 50,    -   k is 10 to 30, wherein the sum    -   m+k is in the range 20 to 60, preferably 20 to 40,    -   -(A¹O)_(n)— represents either a homopolymer of one of the        alkylene oxides mentioned or else block copolymers or copolymers        with random distribution of two or more of the monomers in the        polymer molecule,    -   the units    -   [ ]_(m) and [ ]_(k) may also be present as block copolymers or        copolymers with random distribution of two or more of the        monomers in the polymer molecule.

Sodium, potassium, calcium or magnesium ions are preferably used as themonovalent or divalent metal cations M. The organic amine groups usedare preferably substituted ammonium groups which are derived fromprimary, secondary or tertiary C₁- to C₂₀-alkylamines, C₁- toC₂₀-alkanolamines, C₅- to C₈-cycloalkylamines and C₆- to C₁₄-arylamines.Examples of appropriate amines are methylamine, dimethylamine,trimethylamine, ethanolamine, diethanolamine, triethanolamine,cyclohexylamine, dicyclohexylamine, phenylamine, diphenylamine in theprotonated (ammonium) form.

X may represent —OM_(a) or —O—(C_(p)H_(2p)O)_(q)—R¹ where R¹═H, analiphatic hydrocarbon group with 1 to 20 carbon atoms, a cycloaliphatichydrocarbon group with 5 to 8 carbon atoms, an aryl group with 6 to 14carbon atoms, which may also optionally be substituted. p may have avalue from 2 to 4, q=0 to 100, wherein in a preferred embodiment, p=2 or3, and is thus derived from polyethylene oxide or polypropylene oxide.

Alternatively, X may also represent —NHR² and/or —NR² ₂ where R²═R¹ or—CO—NH₂, which corresponds to the mono- or disubstituted monoamide ofthe corresponding unsaturated carboxylic acid.

Y may be 0 (acid anhydride) or NR² (acid imide).

A copolymer of the general formula IIa or IIb may preferably be used,wherein A¹ is an ethylene group, m is 10 to 30, n is 5 to 20, k is 10 to30 and wherein the sum m+k is within the range 20 to 40.

Compounds of the general formula IIa or IIb may also preferably be usedin which R is an alkyl group with 8 to 18 carbon atoms which isoptionally branched, optionally contains multiple bonds, optionallycontains hydroxyl groups, and A is an ethylene group, M=H or an alkalimetal, a is 1 to 30, b is 1 or 2.

These products may contain compounds of the general formula IIa or IIbin amounts of 0.5 to 30 wt. %, preferably 1 to 15 wt. % or, in total,0.5 to 30 wt. %, preferably 1 to 15 wt. %, of compounds of the generalformula IIa and IIb.

In addition to the components mentioned, further additives and auxiliarysubstances which are known in this field may also be used, if required,such as ethanol, propanol, butanol, propylene glycol, butylene glycol,pentylene glycol, hexylene glycol, alkoxylates, glycol ethers, glycols,polyethylene glycols, polypropylene glycols, Polybutylene glycols,glycerine esterethoxylates, glycerine, polyglycerine, sorbitol, sucrose,fructose, galactose, mannose, polysorbates, starch, xanthan gum,carrageenan gum, cellulose derivatives, alginates, glycol esters,sorbitane esters, opacifiers, solubilisers, ethoxylated fatty alcohols,sodium chloride, sodium sulfate, magnesium sulfate, buffer systems,cholesterol, pantothenic acid, ascorbic acid, polyacrylic acids,carbomers.

The dispersion according to the invention may have a zeta potential ofless than −20 mV in the pH range 4.5 to 7.5. The zeta potential is theoutwardly effective potential of the particles and is a measure of theelectrostatic interaction between individual particles. It plays a partin stabilising suspensions and in particular dispersions with dispersedultrafine particles. When there is a zeta potential of <−20 mV or >+20mV there is a strong repulsion between the particles and the dispersionsremain stable. With values within this range, the repulsion is so lowthat the van-der-waals forces enable the formation of agglomerates andthis leads to undesired sedimentation of the particles.

A zeta potential of less than −20 mV, however, is only one embodiment ofthe invention. The stability of the dispersion according to theinvention is not determined only by an electrostatic interaction.Example 3 describes a stable dispersion (Table 4) with a zeta potentialof only −2.8 mV.

The dispersion according to the invention may also have a viscosity ofless than 2000 mPas at a rate of shear of 100 s⁻¹.

The invention also provides a process for preparing the dispersion whichis characterised in that a stream of a predispersion which containspyrogenically prepared metal oxide particles, each time at least onecompound of the general formula I and/or at least a copolymer of thegeneral formula IIa and/or at least a copolymer of the general formulaIIb, water and optionally additional auxiliary substances, is dividedinto at least two sub-streams, these sub-streams are subjected to apressure of at least 500 bar, preferably 500 to 1500 bar, particularlypreferably 2000 to 3000 bar, in a high energy mill, returned toatmospheric pressure via a nozzle and collide with each other and aremilled in a gas or liquid filled reaction chamber.

High energy mills are commercially available devices. Suitable devicesfor preparing the dispersion according to the invention are, forexample, an Ultimaizer, from the Sugino Co., or the one described inDE-A-100 37 301.

Predispersion can be achieved, for example, using dissolvers,rotor-stator machines or ball mills. The use of rotor-stator machines ispreferred.

The dispersion stream can be circulated in a system so that thedispersion is milled several times using a high energy mill.

Dispersions according to the invention are preferably used to producecosmetic formulations such as make-up, coloured powder, lipsticks, hairdyes, day cremes and in particular sunscreen preparations and may bepresented in conventional forms such as, for example, water-in-oil oroil-in-water dispersions (emulsions), gels, cremes, lotions, sprays.

The dispersions obtained are characterised by the high degree offineness of the dispersed solids, long-term storage stability, lowviscosity and high photostability.

EXAMPLES

The constituents of the dispersion listed in Table 1, apart from theTiO₂ particles, are initially introduced (batch size about 75 kg). Thenthe particles are drawn in through the suction tube of the YstralConti-TDS 3 under shear conditions and the shear process is continued at3000 rpm for a further 15 min after the end of this procedure (sample 0,see Table 2). This predispersion is passed, up to five times, throughthe high energy Sugino Ultimaizer HJP-25050 mill at a pressure of 2500bar with diamond nozzles of 0.3 mm diameter and a sample is taken aftereach passage through the mill (1st passage is sample 1, 2nd passage issample 2 etc.; see Table 2). Table 3 gives the viscosities and Table 4gives the zeta potentials of selected examples. TABLE 1 Formulations (inwt. %) Example 1 2 3 4 5 TiO₂ particles 40 33 35 40 40 Rewophat EAK 8190a 2 2 — 5 — compound of the formula I Guerbet alcohol C₁₂ — — — — 6EO-4-phosphate, a compound of the formula I, b = 1 to 2; M = H Compoundof the — — 21 14 16 formula IIb (MW 15,000) Glycerine — — 10 10 10 Fullydeionised 58.0 65 34 31 28 water

Examples 1 and 3 (Degussa AG): P25; example 2: TN90 (Nippon Aerosil),Examples 4 and 5: T 805 (Degussa AG) TABLE 2 Particle size distribution[in nm]¹⁾ Example Sample 1 2 3 4 5 0 Median 256 216 310 260 236 Mean 289240 335 281 248 Mean/ 1.13 1.11 1.08 1.08 1.05 Median 1 Median 254 148281 174 107 Mean 267 189 318 205 154 Mean/ 1.05 1.28 1.13 1.18 1.44Median 2 Median n.d. 105 229 135 97 Mean 140 268 159 137 Mean/ 1.33 1.171.18 1.41 Median 3 Median 154 n.d. n.d. n.d. n.d. Mean 193 Mean/ 1.25Median¹⁾determined using the Malvern Zetasizer 3000 HSa instrument applyingthe principle of dynamic light scattering. The results are obtained fromvolume-weighted Contin analysis;

TABLE 3 Viscosity of the dispersions [in mPas] as a function of the rateof shear [in s⁻¹] Rate of Example Sample shear 1 2 3 4 5 0 1 66700 287002420 408 34900 100 188 324 4595 266 1400 2 1 5790 31000 359 76 15700 100131 439 130 69 518* The viscosity was determined as the change in viscosity using aPhysica MCR 300 viscometer and the CC27 measuring system

TABLE 4 Zeta potentials¹⁾ in mV at selected pHs and isoelectric points(IEP) Example 1 2 3 4 5 mv/pH −29.7/5.0 −26.3/5.2 −2.8/6.5 −23.4/6.2−22.1/6.4 IEP at <2 <2 2.7 n.p. n.p. pH¹⁾The zeta potential was determined with the DT-1200 instrument fromDispersion Technology Inc., USA.;n.p. = not present (IEPs could not be found over the entire range frompH 2 to pH 10)

The dispersions according to the invention are storage stable for morethan 6 months at room temperature and for more than 1 month at 50° C.

1. An aqueous dispersion comprising pyrogenically prepared oxideparticles of titanium, zinc, iron or cerium with an average particlesize, expressed as the median, in the dispersion of less than 250 nm,wherein the particle sizes of the oxide particles in the dispersion arenot distributed symmetrically and the dispersion comprises, as adispersant, at least one compound of the general formula I

wherein R represents an optionally branched, optionally multiple bond-and optionally hydroxyl group-comprising alkyl group with 6 to 22 carbonatoms, A represents an ethylene group, propylene group, iso-propylenegroup or butylene group, M represents H, an alkali metal or an ammoniumion, a is 0 to 30, b is 0 to 2 and/or at least one copolymer of thegeneral formula IIa

and/or at least one copolymer of the general formula IIb

wherein, for IIa and IIb M represents hydrogen, a monovalent or divalentmetal cation, an ammonium ion or an organic amine group, a is 1 or, inthe event that M is a divalent metal cation, a=0.5 X represents either—OM_(a) or —O—(C_(p)H_(2p)O)_(q)—R¹, where R¹═H, or an aliphatichydrocarbon group with 1 to 20 carbon atoms, a cycloaliphatichydrocarbon group with 5 to 8 carbon atoms, an optionally substitutedaryl group with 6 to 14 carbon atoms, p=2to4,q=0 to 100, —NHR² and/or—NR² ₂, where R²═R¹ or —CO—NH₂ Y represents O, NR² A¹ represents anethylene group, a propylene group, an iso-propylene group or a butylenegroup, m is 10 to 30, n is0to50, k is 10 to 30, wherein the sum m+k iswithin the range20 to
 60. 2. The aqueous dispersion according to claim1, wherein the metal oxide particles comprise the oxides of titanium,zinc, iron, cerium, mixtures thereof and mixed oxides of titanium, zinc,iron, cerium, and mixtures thereof with aluminium or silicon.
 3. Theaqueous dispersion according to claim 1, wherein the surface of themetal oxide particles has been modified using organic compounds.
 4. Theaqueous dispersion according to claim 1, wherein said aqueous dispersioncomprises 20 to 60 wt. % of metal oxide particles.
 5. The aqueousdispersion according to claim 1, wherein in compounds of the generalformula I, R represents a fatty alcohol group with 6 to 22 carbon atoms,and a has a value between 1 and
 30. 6. The aqueous dispersion accordingto claim 1, wherein in the phosphate of the general formula I, Rrepresents a guerbet alcohol group with 6 to 22 carbon atoms, and a hasa value between 1 and
 30. 7. The aqueous dispersion according to claim1, wherein said aqueous dispersion comprises 0.5 to 30 wt. % of thephosphate of the general formula I.
 8. The aqueous dispersion accordingto claim 1 wherein in compounds of the general formula IIa or IIb, A¹ isan ethylene group, m is 10 to 30, n is 5 to 20, k is 10 to 30 andwherein the sum m+k is within the range 20 to
 40. 9. The aqueousdispersion according to claim 1, wherein in compounds of the generalformula IIa or IIb, R represents an optionally branched alkyl group with8 to 18 carbon atoms which optionally comprises multiple bonds andoptionally comprises hydroxyl groups and A is an ethylene group, M=H oran alkali metal, a is 1 to 30, b is 1 or2.
 10. The aqueous dispersionaccording to claim 1, wherein said aqueous dispersion comprises 0.5 to30 wt. % of compounds of the general formula IIa or IIb or in total 0.5to 30 wt. % of compounds of the general formula IIa and IIb.
 11. An Theaqueous dispersion according to claim 1, wherein said aqueous dispersionfurther comprises auxiliary substances and additives.
 12. The aqueousdispersion according to claim 1, wherein said aqueous dispersion has azeta potential of less than −20 mV in the pH range 4.5 to 7.5.
 13. Theaqueous dispersion according to claim 1, wherein said aqueous dispersionhas a viscosity of less than 2000 mPas at a rate of shear of 100 s⁻¹.14. A process for preparing the dispersion according to claim 1, whereina stream of a predispersion which comprises pyrogenically prepared metaloxide particles, each time at least one compound of the general formulaI and/or at least a copolymer of the general formula IIa and/or at leasta copolymer of the general formula IIb, water and optionally additionalauxiliary substances, is divided into at least two sub-streams, said atleast two sub-streams are subjected to a pressure of at least 500 bar,in a high energy mill, returned to atmospheric pressure via a nozzle andcollide with each other and are milled in a gas or liquid filledreaction chamber.
 15. The process according to claim 14, wherein thedispersion is milled several times using a high energy mill.
 16. Amethod of preparing a cosmetic formulation, said method comprisingpreparing said cosmetic formulation which comprises said aqueousdispersion as claimed in claim 1.